Electric heating device

ABSTRACT

An electric heating device especially for fluids, such as liquid fuel, includes a heat element with an electrical resistance. The heat element can be connected to an electrical power source and also includes a carrier element to which en electrically conductive heat layer of a PTC material with positive temperature resistance co-efficient is applied. The heat layer can, at least in two interspaced positions, be connected to the electrical power source.

BACKGROUND

The present invention refers to an electrical heating device, inparticular, for fluids such as liquid fuel, with a heating element whichcomprises an electrical resistance and which can be connected to anelectrical power source.

One such device is known in the market. In it, a so called “heatingcartridge” is arranged in a fuel line, such as an oil supply line for anoil burner. The cartridge comprises a heating coil which has athermo-element and extends into the fuel stream that is transportedinside the fuel line. The thermo-element is used to record and regulatethe temperature. If an electrical voltage is applied to the heatingcartridge, it and the fuel running past will expand. The advantage ofsuch a heating process is that the heated fuel has a low viscosity andtherefore forms a very thin fuel film on the wall of the customaryhollow-cone nozzle. This in turn reduces the flow even when the nozzleopening is relatively wide. A relatively wide nozzle opening isdesirable to prevent plugging.

In this manner, low performance atomizing burners can be used with arelatively wide nozzle diameter. Furthermore, due to the low viscosity,the nozzle atomizes the fuel more evenly and into a finer spray, thusimproving ignition. The overall effect of heating the fuel is areduction in fuel consumption.

However, the device known in the market has the disadvantage that itsdesign is complicated, and it is therefore difficult to integrate itinto the oil supply line. Furthermore, regulating with a thermo-elementand an electronic regulator and control unit is relatively complex,which increases the cost. Finally, in many cases, the heating coil forheating the fuel has to become quite hot, which causes the fuel that isin direct contact with the heating coil to evaporate and to form aninsulating vapor lock between the remaining liquid fuel and the heatingcoil. This thermally insulating vapor lock reduces the heat transferbetween the heating coil and the fuel, which means that more energy isrequired for heating the fuel, and the fuel is not evenly heated.

Another electrical heating device of the above named type that is knownin the market is used, for example, as a hot plate. In this device, too,the temperature of the hot plate is recorded by a thermo-element andadjusted via a separate regulator and control unit. This is alsotechnically complex and, in addition, the reaction is sluggish.

It is therefore the object of the present invention to further develop aheating device of the above named type such that it is easy tomanufacture and easy to integrate, that its technical complexity isreduced, that the fuel can be heated evenly, and that a quick reactiontime can be achieved for temperature regulation.

SUMMARY

This object is achieved by means of the invention in that the heatingelement comprises a carrier element to which an electrically conductingheating layer made of PTC material with a positive resistancecoefficient is applied, and that the layer can be connected to theelectrical power supply in at least two separate places.

When PTC material is used, it becomes completely unnecessary to providean external regulator, since the specific resistance of this materialchanges according to the temperature; that is, as the temperature drops,the specific electrical resistance of the material decreases, whichcauses the current that passes through it to rise, provided that thevoltage is constant. This in turn causes a higher heating output and arise in temperature, with the opposite effect on the resistance.Therefore, such a material is self regulating. To set a certaintemperature, it is only necessary to provide a constant voltage, whileneither a regulator nor a thermo-element is necessary.

Furthermore, due to the small mass of the heating layer, the heatingdevice according to the invention can be very rapidly heated up, and itsoverall dynamics can be distinctive. This in turn leads to the highefficiency of the device. The maximum temperatures that can be achievedwith the heating device are approximately between 500 and 1000° C.

In a first aspect of the invention, an automatic limit can beestablished for the temperatures that can be achieved with the heatingdevice in that a PTC material is used whose temperature resistancecurve, starting at a certain temperature, changes its gradient in theform of a “kink.” (See FIG. 2). As shown, the gradient of thetemperature versus the resistance changes at this kink and thus with anincrease in temperature, the corresponding increase in resistance isless, or limited, after this kink.

In this, it is particularly preferred when the gradient change is formedsuch that a certain temperature can be held constant. In that case, thelocal temperature, which is at least partly determined by thecharacteristics of the material, is “self-adjusting”, and a temperaturesensor and an associated control and/or regulator device is notabsolutely necessary. This self-adjustment can also be a reliable meansto prevent overheating. The fact that additional components can beeliminated considerably simplifies the manufacture of the heating deviceaccording to the invention.

However, it is also possible for the electrical heating device tocomprise an electronic control and/or regulating device which can beused to set a certain temperature in at least one section of the heatinglayer. In this, the heating layer itself can be used as a temperaturesensor, since its resistance is a measure of the temperature. Theresistance of the heating layer can be evaluated in the control andregulating device, and the appropriate temperature can be determined.

Advantageously, the heating layer is applied by means of a thermalprocess, in particular by means of plasma deposition, plasma spraying,high-speed flame spraying, etc. This is provided in another aspect ofthe invention. On the one hand, such a thermal process is cost-effectiveand, on the other hand, it allows optimal bonding between the layers andthe carrier element and also between the layers themselves.

Another aspect is characterized in that the heating layer comprises aceramic powder. This makes manufacturing easy and provides the layerwith special thermal stability. The heating layer may also comprise ametal powder, which improves its application to the carrier element.

In the other aspect of the invention, the carrier element is made of anelectrically conducting material, and an electrically insulating layeris provided between the heating layer and the carrier element. It isoften better to use carrier elements made of metal because they areeasier to manufacture, but metals are electrical conductors. Therefore,in such cases, the heating layer must be electrically insulated from thecarrier element by a layer. Thanks to this aspect, the heating deviceaccording to the invention can therefore be used with the customarymetal carrier elements.

In this aspect, the electrical conductivity of the carrier element canbe used to channel the electrical output to or from the heating layer.This is possible when, as provided in another aspect the electricalpower source is a low voltage source and the heating layer iselectrically connected in one place with the carrier element.

A layer that electrically insulates the heating layer from the carrierelement is unnecessary if the carrier element itself is made of anelectrically insulating material. Such materials include, in particular,many temperature-resistant plastics, but also ceramic materials andglass. In that case, higher voltages can also be used as the electricalpower supply for the heating layer without the necessity of anelectrically insulating layer. As a general principle, the heatingdevice according to the invention can be operated with voltages startingat about 1.5 V, although voltages up to 220 V and more are possible aswell.

User handling of the device according to the invention made easier by anaspect in which an electrically insulating layer is provided on the sideof the heating layer that is opposite the carrier element. This layerprotects the user to come in direct contact with the live heating layer.

One example for a preferred material for the electrically conductinglayers is aluminum oxide, Al₂O₃, and another is zircon oxide. As ageneral principle, at least the insulating layer between the heatinglayer and the carrier element should be a good electrical insulator, buta poor thermal insulator. Furthermore, the material should betemperature-resistant and able to follow the thermal expansion movementsof the carrier element. This is the case with aluminum oxide and withzircon oxide.

Good bonding of the insulation layers is achieved when they are appliedby means of a thermal process in particular plasma deposition, vapordeposition or highspeed flame spraying.

One aspect of the heating device is characterized in that the thicknessof the heating layer varies over the length of the carrier element, suchthat the output distribution or output absorption varies also over thelength of the carrier element. In this manner, a certain temperatureprofile can be achieved in the two directions of the carrier element'splane without the necessity of a complex regulator or control device.The variation in thickness can be continuous, which would also allow fora continuous output distribution.

In this, it is particularly preferred if, during operation, thetemperature difference between the carrier element and the material tobe heated can be held constant over the length of the carrier element.This would take into account the fact that the temperature of thematerial to be heated may change over the length of the carrier element,e.g., colder temperature at the edge. By varying the temperature of theheating layer over the length of the carrier element, the heating upprocess can be optimized, and, if necessary, the length of the heatinglayer necessary for heating can be reduced.

The thickness range of the layers can be selected to be optimal.Accordingly, the thickness of at least one of the layers is in the rangeof 0.002 to 0.2 mm, but preferably in the range of 0.005 and 0.1 mm. Asfar as the heating layer is concerned, layers of such thickness form aresistance that is necessary for achieving the required temperatures inthe range of up to 400° C. On the other hand, with regard to theelectrically insulating layers, a thickness in the above range ensuresthat the thermal insulation effect is as low as possible.

Furthermore, the aspects of the invention name a number of especiallypreferred applications. Thus, the heating device can preferably be usedfor the heating of oil supplied to a burner, as an instantaneous waterheater, as a hot plate, for heating the coolant in automobiles, forheating fuel filters for paraffin precipitation, for heating windshieldsor mirrors, for the de-icing of aircraft wings, for heating the walls ofrooms or for heating floors, e.g., to prevent freezing, or as a warmingplate. The heating device according to the invention can be applied toany regular or irregular surfaces with any geometry, and also to unevenand/or rough surfaces. It is possible to apply it cost effectively witha robot.

An aspect of the invention in which the carrier element comprises atubular element is suitable for the heating of fluids.

In a particularly preferred aspect of the invention with a tubularcarrier element, the element comprises a fuel line with an inlet and anoutlet, whereby at least some parts of the heating layer are applied toa wall of the fuel line. Thus, a heating coil or the small contactsurface between the same and the fuel are completely unnecessary in thedevice according to the invention. Instead, at least one section of thefuel line wall is heated by the heating layer. The contact surface thuscreated between the heated fuel line wall and the fuel itself isconsiderably larger than the contact surface between the fuel and aheating coil, which means that the temperature of the wall itself can belower, thus reducing the danger of fuel vapor formation. Of course, themore the surface of the fuel line wall increases, the more apparent thisadvantage becomes. It is therefore best if the places where the poles ofthe electrical power source are connected with the heating layer areseparated from each other as far as possible, for example, one at theinlet and one at the outlet.

In one aspect, the fuel line has an injection nozzle at one end. Thishas the advantage that the path from the heated area to the injectionnozzle is relatively short. If need be, the injection nozzle can also beheated as well by means of an appropriate coating, which in additionpromotes the formation of an optimal fuel spray.

In a particularly preferred aspect, the fuel line, at least in the areaof the heating layer, comprises an annular chamber through which thefuel is led. In this manner, the actual fuel volume to be heated isreduced, thus, on the one hand, reducing the required heat output and,on the other hand, improving temperature distribution in the fuel thatflows through the fuel line, as well as shortening the regulatingreaction time.

Furthermore, a contact means for the heating layer should be providedthat is particularly easy to handle. According to it, the device isprovided with at least one contact ring that can be pulled onto the fuelline and establishes an electrical contact with the heating layer. Forexample, such a contact can be made via a blade which, when pulled on,scores an electrical insulation layer that may exist on the heatinglayer and digs into the heating layer, thus ensuring a safe contact.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a side partial cross-sectional view of a heating device inaccordance with the principles of the present invention; and

FIG. 2 is a graph of a temperature versus resistance curve illustratinga kink in accordance with the principles of the present invention.

DETAILED DESCRIPTION

In the drawing, a device for the heating of fuel is identified by theoverall reference number 10. The device 10 is shown with partialsections cut out and comprises a tube which at the end which is at theleft end is provided with an outlet 13 and which is closed by a nozzlepiece 14. Nozzle piece 14 is welded into tube 12. Tube 12 is open at itsright end, thus forming an inlet 11. At this open end, it can beprovided with a thread not shown, to connect it to a fuel supply. Nozzlepiece 14 is provided with a central nozzle opening 16, whose crosssection tapers toward the outside, through which the fuel is dischargedfrom the tube 12 during operation, and a fuel spray is generated.

A hollow displacer 18 is provided coaxially to the tube 12 and in theinterior of said tube. The left end of the displacer 18 is supported byfeet 20 in associated recesses 22 in the nozzle piece 16. At its rightend, it is fastened to the interior wall of tube 12 by a washer 24.Along its entire circumference, this washer 24 is provided with holes 26through which during operation the fuel can flow into an annular chamber28 formed between the displacer 18 and tube 12.

Tube 12 is a conventional steel tube, such as is used for fuel lines,e.g., for oil lines leading to heating-oil burners, etc.

On its radially outside casing surface, an electrically insulating layer30 of aluminum oxide is applied by means of plasma deposition. Thethickness of the electrically insulating layer is about 0.1 mm. To theelectrically insulating layer 30, a heating layer 32 is applied by meansof powder plasma deposition, but this layer, in contrast to theinsulating layer 30, does not extend over the entire length of tube 12,but ends at a distance a before the two ends of tube 12.

From end 11 at the outlet to end 13 at the inlet, the thickness of theheating layer 32 decreases from about 0.1 mm to about 0.05 mm. Thematerial of heating layer 32 is a nickel, chromium and iron alloy with apositive temperature coefficient PTC. The nickel, chromium and ironalloy is embedded in a base of powdery ceramic material. An outerelectrically insulating layer 34 is applied in turn to heating layer 32,also by means of plasma deposition. This layer 34 also consists ofaluminum oxide and is separated by a distance b from the two ends oftube 12. This distance b is slightly greater that distance a, whichmeans that the two ends of heating layer 32 are free.

At the two ends of tube 12, i.e., in separate places, contact rings 36,38 are pulled onto the tube 12 and its layers 30, 32 and 34. The twocontact rings 36 and 38 are made of an electrically insulating material,such as plastic. The radially inside annular surface of the contactrings 36 and 38, which faces tube 12, is slightly graduated to take intoconsideration that the outer electrically insulating layer 34 ends by adistance b before the end of pipe 12 and the heating layer 32 by adistance a. Contact pins 40, 42 are inserted into contact rings 36, 38.These contact pins 40, 42 are made of an electrically conductingmaterial; on the one side the contact pins 40, 42 contact heating layer32 and, on the other side, the contact pins 40, 42 can be connected to avoltage source 44 via an electrical line 45 and a control unit 46.

Device 10 functions as follows: Via a pump, not shown, and via a supplyline, also not shown, a fuel, such as heating oil, is conducted throughtube 12 from inlet 11 to outlet 13 as shown by arrows 48. In tube 12,only annular space 28 between displacer 18 and the radially insidecasing surface of tube 12 is available in the area of the lengthwiseextension of heating layer 32. At this point it should be noted thatneither displacer 28 nor heating layer 32 or insulating layers 30, 34necessarily have to extend over the entire length of tube 12. However,the larger the contact surface between the heated wall and the fuel, themore heat is generated for the fuel.

When control unit 46 causes the closing of the circuit formed by voltagesource 44, line 45, control pins 40, 42 and heating layer 32, heatinglayer 32 occurs between contact rings 36, 38, i.e., substantially overthe entire length of tube 12. Due to the fact that heating layer 32 isthicker at inlet end 11 than at outlet end 13, more heat is generated atthe former than at the latter. The electrically insulating layer 30transfers the heating of heating layer 32 through tube 12, such thattube 12 is substantially heated as desired over its entire length andits entire circumferential area. If the right kind of material is chosenfor nozzle piece 14, this ensures that as the result of heat transfer,nozzle piece 14 and nozzle opening 16 are heated as well.

The fuel, on the way from the inlet end 11 to the outlet end 13, passesthe heated inner wall of tube 12 in the direction of arrow 48 and isthus also heated along its flow path. Due to the fact that the thicknessof heating layer 32 differs from one end to the other, the temperatureof tube 12 also rises from inlet end 11 toward outlet end 13, such thatthe temperature difference, which determines the heat transfer betweentube 12 and fuel 48, can be held substantially constant throughout theflow path of fuel 48. In this manner, a large amount of thermal energycan be injected into fuel 48 over a relatively short flow distance.

Insulation layers 30, 34 reliably protect the operators who are handlingdevice 10 from contact with live elements. Furthermore, the material andthe thickness of the outer insulating layer 34 can be selectivelychosen, which means that this layer provides thermal insulation as well,thus reducing the energy requirement even more. It will also beappreciated that instead of a D.C. current source, an A.C. currentsource of higher voltage can be used as well.

Furthermore, in an aspect not shown, the radially outside casing surfaceof displacer 18 is also provided with insulating layers and a heatinglayer, which means that the two boundary walls of the annular space thusformed can be heated, causing the fuel to be heated even moreefficiently.

In another aspect not shown, there is no contact ring at one end of thetube. Instead, the inner electrically insulating layer, i.e., the layerwhich electrically insulates the heating layer from the tube, isslightly recessed at that end, which means that there is electricalcontact in that place between the heating layer and the tube. In thatcase, which for safety reasons is naturally possible only with a lowvoltage source, the tube can be used as one of the two supply lines forthe electrical output.

1. An electrical heating device comprising: a heating layer applied by athermal process on a tubular carrier element comprising an inlet and anoutlet, the heating layer comprising a PTC material having a temperatureresistance curve defining a kink, wherein the kink provides an automaticlimit to maintain a constant temperature; an injection nozzle disposedproximate the outlet of the tube; and a displacer disposed within thetubular carrier element such that an annular chamber is formed betweenthe tubular carrier element and the displacer for allowing passage of afluid to be heated, wherein the fluid does not flow inside thedisplacer.
 2. A method of forming an electrical heating device, themethod comprising the steps of: applying a heating layer to a carrierelement using a thermal process selected from a group consisting ofplasma deposition, vapor deposition, high-speed flame spraying, andplasma spraying, the heating layer comprising a PTC material having atemperature resistance curve defining a kink, wherein the kink providesan automatic limit to maintain a constant temperature; applying aninsulating layer over the heating layer using a thermal process; andengaging a contact ring over the insulating layer such that the contactring scores the insulating layer to make electrical contact with theheating layer.
 3. The method according to claim 2 further comprising thestep of applying an insulating layer to the carrier element using athermal process before the step of applying the heating layer.
 4. Themethod according to claim 3, wherein the thermal process used to applythe insulating layer to the carrier element is selected from a groupconsisting of plasma deposition, vapor deposition, high-speed flamespraying, and plasma spraying.
 5. The method according to claim 2,further comprising varying a thickness of the heating layer along alength of the carrier element such that an output distribution of poweralso varies over the length of the carrier element.